fish

A singular Amazonian catfish is capable of an amazing feat: hidden from human eyes, the species travels vast distances over its lifetime, making a round trip covering more than 8,000 kilometers (nearly 5,000 miles), to return to its natal breeding grounds, a new study confirms.

But even as this record-setting feat — the longest freshwater migration in the world — is scientifically confirmed, the species is threatened by hundreds of planned Amazonian dams.

Brachyplatystoma rousseauxii is a commercially valuable catfish species and an apex predator, growing to 3 meters (more than 9 feet) long. Understanding the migratory patterns of the fish, whose range spans six Amazonian countries, “is paramount for designing adequate conservation and management strategies, especially in view of the current and proposed hydroelectric development throughout the Amazon basin,” the researchers write in the Journal of Applied Ecology.

Observations in the 1990s concerning the size distribution of catfish caught along the length of the Amazon River first led researchers to suggest that long-distance homing migration might be taking place, explained study lead author Fabrice Duponchelle of France’s Institute of Research for Development. Subsequent genetic analyses were consistent with that hypothesis, but still didn’t offer definitive proof.

Intrigued by the possibility that catfish might be homing over such vast distances, Duponchelle employed an innovative technique in the new study to get conclusive evidence: chemical analysis of the otolith, a type of ear bone.

As otoliths grow, their chemical composition changes to reflect the background levels of particular chemical elements found in the environment. Like tree rings, the layers of bone relate to their age: the center, innermost layer of otolith is the oldest, and reflects life as a hatchling; the outer edge is the most recent, and reflects the last stage of the fish’s life.

The giant Amazonian catfish is a valuable commercial species, an apex predator, and the world’s long distance freshwater fish migration record holder. Photo courtesy of the USGS Columbia Environmental Research Center.

Imagine a fish isolated in an Amazonian lake — part of the vast freshwater ecosystem of the Amazon basin, an ever-changing network of rivers, lakes and floodplains that extends to 1 million square kilometers (386,102 square miles).

Now imagine that isolated fish as water levels rise during the wet season, and floodplains vanish beneath up to 15 meters (49 feet) of water. The fish — once restricted by the lake’s edge — swims freely into the flooded forest and mingles with others of its kind from elsewhere.

For thousands of years, isolated fish populations across the Amazon have likewise played a game of musical chairs: intermixing between flooding water bodies, migrating short and vast distances between lakes and along river channels, and then as the waters receded, forming new lake and river populations.

This connectivity — with the genetic mixing it affords — is vital for healthy fish populations, but is extremely vulnerable to changes in the annual “flood pulse” that inundates forests.